can some one explain me which partition of my ubuntusystem I should make big for all the Data/Files/Database etc...the openkm DMs contains.
It's only to not run out of space after the installation.
It's always good to know where the standard path are of such a installation.

OpenKM use a database and also a datastore. Usually the database are few GB ( consider less 4GB ). If you use Mysql then the database will store files by default at /usr/lib/mysql ( you can move these files to another location ).

Do this questions:
- How many files I have. How many I create during a year. Which is the estimated size. Then year * 1.3 * 5 ( set 30% and 5 years estimation). This is the disk space you will need. If the disc comes full, you can easilly move to another location as I explained before.